Water changes eliminate the guess work involoved in dosing or build up of many things you cannot test for or have test kits for.
Each tank changes it's uptake, needs of nutrients, ratios of one nutrient to another etc through time. A water change essentially "re sets" the tank.
To mimic stable levels, large water changes with good dosing allows for maintenance of good plant nutrient levels with minimal testing and guess work. No bad nutient levels ever build up this way for very long nor at very high levels.
You basically can re set your tank and make a "reference" solution each water change for the 50% etc that you change each week.
Good basic maintenance like large frequent water changes will help the critters and plants in any tank for that matter(SW, reef, FW fish only etc).
Think about it like this, 4 people living in a bathroom and only flushing the toilet once a week vs daily. Water changes are like the flushings.
Regards,
Tom Barr